High-Definition Multimedia Interface (HDMI) has become the prevalent specification for transmitting digital video and audio data from high bandwidth data sources to digital data presentation devices. HDMI sources such as DVD players, Blu-ray disc players, personal computers, set-top boxes, video game consoles, etc., output video and audio data generated from these sources according to the HDMI specification. Digital data presentation devices capable of receiving the HDMI data are called HDMI sinks, and examples are digital televisions that may be either high-definition, non-high definition, computer monitors, laptop computers, video game consoles, home theater audio/video receivers, or other devices.
The HDMI audio and video data is transmitted with an accompanying low bandwidth communication channel, referred to as a “display data channel” or “DDC.” The DDC has two lines: Serial Data (SDA) and Serial Clock (SCL). The two DDC lines are used to allow the HDMI source to learn about the HDMI sink's capabilities, such as screen resolution or other features, as well as for authentication when encrypted audio/video data is transferred. The DDC connection is implemented in the HDMI specification using the “inter-integrated circuit” or I2C bus specification.
The data within the DDC is Extended Display Identification Data (EDID) and High-bandwidth Digital Content Protection (HDCP) data. The EDID, which indicates the HDMI sinks capabilities, can include manufacturer name, product type, phosphor or filter type, timings supported by the display, display size, luminance data and pixel mapping data. The HDCP data is a proprietary data stream that provides copy protection to the data supplied by an HDMI source to an authenticated HDMI sink. The HDCP copy protection data is a series of keys as well as calculation results based on the series of keys that are exchanged between the HDMI source and HDMI sink. Since the EDID data is related to the hardware of the sink, it remains substantially static. In contrast, parts of the HDCP data are continuously changing because the HDMI source is continuously verifying the HDMI link.
Presently, HDMI cables connecting HDMI sources with HDMI sinks are formed from twisted-pair, copper wires that provide satisfactory connection lengths up to approximately 5 meters. The high data rates of HDMI and the line capacitance limitations of I2C bus specification limit the HDMI cables from extending beyond the approximate 5 meters. Note: There are ‘adaptive equalizers’ that can allow a HDMI-TX to drive up to 10 meters.
Optical cables are seen as a viable solution to overcoming the distance limitations of the present twisted-pair cable and the bandwidth limitations of wireless communication techniques. Of course, other cables, such as coaxial cables, may used in alternative embodiments of the disclosed invention since most practical cables suffer from the following limitations. The data, including I2C data, provided from the HDMI source can be serialized and sent over the cable in a serial data stream. However, the I2C datagram is a bi-directional datagram in which data sent from an HDMI source to an HDMI sink requires the exchange of data (i.e., acknowledgements and responses to data requests) between the HDMI source and HDMI sink, the uni-directional nature of the serial data stream over the cable must be interrupted to allow for the returning response or acknowledgement. This interruption introduces a delay to facilitate the I2C data exchange. This delay and the facilitation of the I2C data exchange can be addressed in a number of ways.
For example, the I2C specification defines a “clock stretching” technique to accommodate communication delays between a source and a sink, if, for example, the source requests a read of data that the sink cannot satisfy immediately. The HDMI specification states that all HDMI-compliant sources shall support this feature. Market research indicates, however, that some vendors have built equipment that, although they otherwise support HDMI protocols, do not support the clock stretching feature. These HDMI sources that do not comply with the HDMI specification can be considered HDMI compatible. In cases where a HDMI source does not support clock stretching while the device it is connected to does rely on the presence of the clock stretching functionality, communication is likely to be disrupted.
Data mirroring is another method of addressing the data latency associated with the exchange of data between the source device and the sink device. With data mirroring, data is read from the sink device and stored in an integrated circuit at a head end of an optical cable, closest to the source device. Specifically, EDID data, which represents a sink device's capabilities, and HDCP data, which supports the authentication operations for encryption and data rights management are stored in memory at the head end. Although, data mirroring attempts have been proposed for use with HDMI interconnects, all known attempts have been deemed non-compliant by HDMI ratification bodies. Therefore, there is a need for an optical cable that provides the HDMI specification-compliant operation of ‘clock stretching’ to those HDMI sources that support it, while at the same time offer the operational benefits to those HDMI sources that do not implement all of the features, such as clock stretching, available under the HDMI specification.
The Digital Visual Interface (DVI) specification is directed to providing high quality digital video for displays, projectors and monitors. Devices that provide DVI data would also benefit, and be suitable for use with the disclosed embodiments, or variations thereof.